LONDON – With a week still to go, Wimbledon’s grass courts already are not the lush lawns players — and TV viewers — are accustomed to seeing early in the tournament.

Novak Djokovic noticed the ball bouncing differently in some patches.

Roger Federer and his opponent slipped during their third-round match.

The two past champions are wondering what sort of work can be done on the middle Sunday, when there is no competition, so the court conditions improve when action resumes with all 16 fourth-round singles matches scheduled for Monday.

“The first two matches I didn’t see any significant difference. But I was hearing a lot of comments from the other players. They were complaining. Especially on the outside courts,” Djokovic said after beating Ernests Gulbis in straight sets at Centre Court on Saturday.

“Today, I could see there is a difference in grass, in (the) turf itself. It was a bit softer, I would say, especially around a couple of feet inside and outside, around the baseline area,” said Djokovic, a three-time champion at the All England Club. “I haven’t had that kind of experience before in Wimbledon, to be honest. I mean, the courts are always perfect here.”

Federer followed Djokovic on Centre Court and described it as “a tad slippery” after losing his footing during one point in a victory over Mischa Zverev, who also took a tumble.

“But I didn’t feel like it was unsafe,” said Federer, who has won seven of his record 18 major titles at Wimbledon. “Maybe you don’t want players to feel that way, because the moment you become scared of moving properly, it’s really difficult to play, I must admit. The last thing we want to see is horrible injuries.”

Some players voiced concerns about the court conditions over the opening week, including Kristina Mladenovic of France and Alison Riske of the United States, who each fell early during their second-round match on Court 18 on Thursday.

Riske, who won the match, said afterward that the dirt “is like ice.”

Measurements taken throughout Wimbledon’s first week show the grass courts are as healthy as they have been in recent years, according to Neil Stubley, the head of courts and horticulture at the All England Club.

“We looked at the baselines and the areas that they thought there was an issue,” Stubley said. “We didn’t feel there was. The Grand Slam supervisor and assistant referee didn’t believe that there was either. When you look at the comparisons of other courts ... they were in or around the same condition as the other courts.”

Very little rain has fallen over the first six days of the tournament, which Stubley said led to more wear on the courts.

Still, although he acknowledged the courts are under stress, he said there’s “not a doubt” they will hold up throughout Wimbledon’s second week.

“Because we have daily monitoring, we can keep a very tight rein on everything,” he said. “It never gets to the point where it’s ever going to get away from us because we’re on top of it every single day.”

Still, Djokovic was sure that something wasn’t quite the same as in the past.

“I’m sure they know what they’re doing. They know their job the best in the world. But you can see that there is a slight difference in the quality of the grass,” he said. “This year seems like it’s a bit softer.”

Noted Federer: “They have time to figure out how they’re going to prep (the courts) for Monday.”

Associated Press writer Zac Boyer contributed to this report.